Photovoltaic (PV) cells, also referred to as solar cells, are one type of optoelectronic device used to produce electrical energy from sunlight. These solar cells are built from various semiconductor systems which must be protected from environmental effects such as moisture, oxygen, and ultraviolet (UV) light. The cells are usually packaged on both sides by encapsulating layers of glass and/or plastic films forming a multilayer structure known as a photovoltaic module.
One important consideration in developing cost effective thin film PV modules is the ability to use low cost commodity polymers such as transparent polyolefins, poly(meth)acrylates, or the like as PV module encapsulant materials. However, transparent polyolefins and poly(meth)acrylates generally do not meet the long-term requirements of the application (at least 20 years) with respect to outdoor weatherability. The failure is due to the aging effects of water, oxygen, UV exposure and temperature.
Manufacturers have added UV radiation absorbers to polymer films in an effort to protect against UV degradation and improve long-term weatherability. However, high concentrations of ultraviolet radiation absorbers typically result in decreased mechanical performance of the polymer film. Higher concentrations also can cause haze within the polymer film, which reduces the amount of light captured by the devices. In addition, previous investigators have noted leaching of UV absorbers from polymer films. Underlying substrates or components may be damaged as the protective film degrades. This, of course, leads to decreased performance over time. As a result, the lifetime of the polymer films and devices incorporating these films may be limited. As such, improved weatherable polymer films are desired. Fluoropolymer compositions are widely recognized as a useful component as a potential top layer in photovoltaic modules due to their excellent surface properties (e.g., low surface tension that provides stain resistance and repels dirt), optical properties, electronic properties, and weatherability. Since UV radiation will affect the physical properties of materials under the fluoropolymer protection layer, adding the UV blocking functionality in the fluoropolymer top protection layer offers the potential to increase the weatherability of underlying layers.
However, the low solubility and/or high mobility of typical ultraviolet light absorbers in fluoropolymers make it difficult to maintain high levels of these compounds in fluoropolymeric films. The compounds tend to migrate within or even out of the film. Migration is particularly problematic. If ingredients are too mobile, the ingredients can migrate within the film as well as from the film. Migrating materials can contaminate other materials or interfaces between materials. This can lead to hazing, delamination, loss of transparency, loss of UV protection, degradation, and/or the like. The protective properties of the compounds may be lost or degraded in areas where migration has occurred. Some kinds of ultraviolet absorbers also are unduly reactive with fluoropolymers, further undermining protection.
Therefore, a need remains for ultraviolet UV protection strategies that are compatible with and resist blooming out of fluoropolymers, and for products such as fluoropolymeric films and other articles that incorporate such protection.